1. Field of the Invention
The present invention relates to a separator for a fuel cell, and more particularly, to a separator for a fuel cell made of stainless steel, tungsten (W) and/or Molybdenum (Mo).
2. Description of the Related Art
Fuel cells produce electrical energy through the electrochemical reaction of fuel with oxygen. The operating mechanism of a fuel cell begins by oxidizing a fuel, such as hydrogen, natural gas, or methanol at an anode in the fuel cell to produce an electron and a hydrogen ion. The hydrogen ion produced at the anode passes through an electrolyte membrane to a cathode, and the electron produced at the anode is supplied to an external circuit through a wire and then is returned back to the cathode. The hydrogen ion combines with the electron and oxygen from the air at the cathode to form water.
Fuel cells may be classified as polymer electrolyte membrane fuel cells (PEMFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), or solid oxide fuel cells (SOFCs) according to the type of electrolyte used. Operating temperatures and the materials used in the elements of the fuel cell vary depending on the type of fuel cell.
PEMFCs may be operated at relatively low operating temperatures, such as about 80° C. to about 120° C., and have a very high current density. PEMFCs can thus be used as a power supply for vehicles and homes.
PEMFCs may include a bipolar plate and a membrane electrode assembly (MEA). The MEA includes an anode in which the fuel is oxidized, a cathode in which an oxidizing agent is reduced, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane should have an ion conductivity sufficient to deliver a hydrogen ion from the anode to the cathode. The electrolyte membrane also serves to insulate the anode from the cathode.
The bipolar plate may include channels through which fuel and air flow. The bipolar plate also functions as an electron conductor for transporting electrons between MEAs. The bipolar plate should be non-porous to keep the fuel and the air separated, and should have excellent electrical conductivity and sufficient thermal conductivity to control the temperature of the fuel cell. Furthermore, the bipolar plate should have a mechanical strength sufficient to bear a force clamping the fuel cell together and should be corrosion resistant in the presence of hydrogen ions.
In the past, PEMFC bipolar plates were usually made of graphite, and the fuel and air channels were usually formed by milling. Graphite plates generally have sufficient electrical conductivity and resistance to corrosion. However, graphite plates and the milling process are very expensive. Furthermore, graphite plates are brittle, and it is therefore difficult to process graphite bipolar plates less than 2 to 3 mm thick. Due to the thickness of the graphite bipolar plates, the fuel cell stacks cannot be made sufficiently thin, especially when the fuel cell stacks include several hundred unit cells.
To reduce production costs and the thickness of the bipolar plates, attempts have been made to produce a bipolar plate made of metal. Metals have most of the physical properties required for the bipolar plates and raw material and processing costs for metals are relatively low.
However, metallic bipolar plates may corrode under acidic conditions inside a fuel cell, and an oxidized film may form, which may result in membrane poisoning and increased contact resistance. Corrosion of the metallic bipolar plate may also poison the catalyst and the electrolyte due to the diffusion of metal ions into the electrolyte membrane. Poisoning decreases the activity of the catalyst and reduces the proton conductivity of the electrolyte, which results in the deterioration of the performance of the fuel cell. In addition, as corroded metal is removed from the metallic bipolar plate, the contact between the separator and the MEA deteriorates and increases electrical conductivity resistance, which degrades the performance of the fuel cell.
Attempts have been made to coat metallic bipolar plates with materials that have anti-corrosive properties and good electrical conductivity. Korean Laid-Open Patent Publication No. 2003-0053406 describes coating a bipolar plate composed of Ti or stainless steel with a TiN alloy. However, in a 1,000-hour performance test, a PEMFC including a bipolar plate made of stainless steel, a Ti alloy, or an Ni alloy does not perform as well as a PEMFC including a graphite bipolar plate.